Add a method to RpcModule that transforms the module into a `RpcModule<()>`,...

Add a method to RpcModule that transforms the module into a `RpcModule<()>`, i.e. removes the context. (#660)

* Add a method to RpcModule, `decontextualize`, that transforms the module into a `RpcModule<()>`, i.e. removes the context.

* Merging a module with an empty module cannot fail

* fmt

* Address grumbles

* Cleanup macro

* Update core/src/server/rpc_module.rs

Co-authored-by: Maciej Hirsz <[email protected]>

Co-authored-by: Maciej Hirsz <[email protected]>